Hydrided lithium-magnesium alloys and method



United States Patent HY DRIDED LITHIUM-MAGNESIUM ALLOYS AND METHOD David W. Lillie and Robert L. Fullman, Schenectady,

'N.Y., assignors to General Electric Company, a corporation of New York No Drawing. Filed Sept. 12, 1957, Ser. No. 683,446

3 Claims. (Cl. 14813.1)

This invention relates to structural alloys of magnesium which contain lithium and more particularly to such alloys in which the lithium is substantially all converted to lithium hydride.

Magnesium and alloys composed of about 80% by weight or more magnesium have had considerable use as structural materials where light weight has been a factor. Various alloying elements have previously been added to magnesium in amounts, in the aggregate, of up to about 20 weight percent or more in order to improve the mechanical properties of commercial, substantially pure, magnesium. In view of present technological advances, particularly in the aircraft field. it has become desirable to improve the mechanical properties of light-weight alloys, such as the magnesium alloys, when they are exposed to temperatures up to about 600 F.

The previously known magnesium-base alloys that have exhibited the more improved mechanical properties at temperatures substantially above room temperature than substantially pure magnesium have been precipitation hardening alloys. In these alloys the magnesium is alloyed with one or more elements which have relatively high solubility in magnesium at high temperatures but low solubility at lower temperatures. The composite alloy is heated to high temperature to dissolve the added element or elements and quenched rapidly to room temperature to produce a super-saturated solid solution of the dissolved elements in the magnesium. Upon reheating to slightly elevated temperatures, precipitation occurs in the form of fine particles, which causes an increase in hardness and strength of the alloy. This heat treatment and accompanying precipitation is known as aging in the art.

If these previously known alloys are exposed to elevated temperatures of about 400600 F. (200 to 320 C.) for prolonged periods of time however, these improved mechanical properties derived from aging decline substantially, a phenomenon known as over-aging. This is the result of the growth of the hardening particles and the depletion of the alloying elements from the matrix by diifusion. For example, a decrease in ultimate tensile strength of as much as 50% has been observed to occur within 500 hours at temperatures of about 500' to 600 F. during such an over-aging occurrence.

Another disadvantage of the previously known magnesium alloys is that the strength advantages obtained by adding alloying elements have been secured at the expense of increasing the density of the final alloy. In structural applications where the weight and volume of a structural member designed to withstand a given load is important, such as in an aircraft design, overaging of such a structure element to the extent that it may lose up to 50% of its strength substantially diminishes the initial advantages which the alloy may have enjoyed.

It would therefore be desirable to provide a structural alloy of magnesium hardened in such a way that it would resist the tendency to overage and would retain its strength at elevated temperatures. Additionally, it would be de- 2,961,359 Patented Nov. 22, 1960 .sirable to provide such an alloy in which the density is lowered by the strengthening alloying-addition, rather than increased as in previous alloys. Yet further, in the field of atomic energy, a light-weight shielding material capable of slowing down high energy neutrons, absorbing thermal neutrons and degrading secondary gamma rays produced by neutron interactions is desirable, particularly in the field 'of nuclear propulsion for vehicles and the like. The magnesium alloys of our invention are efiicient in this shielding function.

It is therefore a principal object of our invention to provide magnesium alloys strengthened by internal hydriding and therefore having improved stable elevated temperature mechanical properties.

A further object of our invention is the provision of such 'an alloy which is readily workable before it is hardened by a hydriding heat treatment.

A yet further object of our invention is the provision of a light-weight workable alloy having good shielding properties against neutron and gamma radiations from a nuclear reaction.

Other and more specific objects of our invention will become apparent from the detailed description which follows.

Briefly stated, in accordance with one aspect of our invention, magnesium and alloys composed principally of magnesium are first alloyed with an amount of lithium equal to from about 0.5 to 30 weight percent, based on the weight of magnesium present. These alloys may be worked into the final desired form and then substantially strengthened by means of the internal hydriding phenomenon by heating in an atmosphere of hydrogen until substantially all the lithium is converted to a stable, strengthening precipitate of finely divided lithium hydride dispersed through an essentially magnesium matrix or, if cast to final form, may be hydrided without the working step.

In order to more particularly disclose our invention, the tensile test results of a number of specific magnesiumlithium alloys which were prepared, worked and heat treated according to our invention follow. These alloys were all prepared from commercially available high purity (about 99.95 to 99.99 percent) magnesium and high purity (about 99.9 percent) lithium in order to eliminate variables caused by significant amounts of impurities. It will be obvious to those skilled in the art, from the nature of the reaction forming the precipitate of lithium hydride, however, that the same principles are equally well applicable to the many known magnesiumrich alloys. For example, such commercially available magnesium alloys have conventionally contained hardening additions of up to about 12% aluminum, up to about 12% zinc, up to about 5% tin, and up to about 1.5% manganese. One such alloy which is conventionally used for castings as well as for wrough articles is conventionally composed of about 10% aluminum, 0.1% manganese with the balance being substantially all magnesium. An alloy containing about 9% aluminum, 2% zinc, 0.1% manganese and the balance magnesium is principally used as a casting alloy while an alloy containing about 5% tin, 3% aluminum, 0.5% manganese and the balance magnesium is useful as a hammer forging alloy. It will be appreciated of course, that there are numerous other well-known magnesium-rich alloys to which our invention may be applied and that the specific alloys named above are only illustrative of base alloys to which up to 30% lithium, based on the magnesium content of the particular alloy may be added, the alloy cast and worked to some final form if desired and then hardened by the internal hydriding of the lithium. Other known alloys have contained up to 5% silver, up to 20% cadmium, up to 1.5 nickel and up to 1.5 copper as we;

alloys if they are to be forgeable.

as minor amounts of various other elements as impuri ties. As is well-known, the sodium content must be maintained below about 0.1% in magnesium-lithium base magnesium, 6.3% aluminum, 1.4% zinc, 0.07% manganese and about 30.4% lithium hydride as a fine dispersion throughout the magnesium alloy matrix.

In the following exemplary alloys, the high purity magnesium and lithium were melted under a helium atmosphere and cast into an iron mold to form ingots about 1 inch in diameter and 2 inches long; These ingots were then swaged to wire 0.040 inch in diameter and annealed under a protective atmosphere of argon at 325 C. for 2 hours and permitted to cool. Tensile tests were performed on specimens in these wires as annealed at 325 C. and upon identical specimens from these same wires after further heat treatment in helium and hydrogen atmospheres and the test results compared as set forth in the following table.

Table l [Treatment 325 0., 2 hours, argon] Test Ultimate Percent Li Temp. Tens. Percent o. E Str. (p.s.i.) Elong [Trcatrnentz 325 0., 2 hours, argon; 500 0., 4 hours, helium] LT 7. 890 14, 000 8. 8 l. 150 5, 940 8, 810 18. 4 250 3, 270 4, 000 30. 7 1T. 12, 000 16, 650 5.0 2 150 6,270 9, 500 19. 6 250 3, 330 3.975 18. 3 '\'.T. 7,900 15, 925 9. 6 4 150 ,205 8,550 24. 6 250 3, 320 3, 745 15. 1

[Treatment: 325 0., 2 hours, argon; 550 0., 4 hours, hydrogen] KT 11,900 10, 600 3.0 1 150 6, 000 9, 320 8.0 250 4, 710 5, 470 14. 2 LT 14, 200 18, 400 2. 2 2 150 8,070 10,400 10.5 250 5, 330 5, 610 13. 3 KT. 14, 000 19, 000 2. 0 4 150 8, 035 10, 965 10. 9 250 5, 010 5, S20 29. 1

[Treatment: 325C., 2 hours, argon; 450 0., 9 hours, helium] R.T. 11, 320 14,130 8. 5 13 150 3, 400 3, 550 28. 6 250 1, 625 l, 675 8.3

[Treatment: 325 0., 2 hours argon; 450 0., 2 hours hydrogen] KT. 9, 650 12, 290 3. 5 13 150 6, 600 7,100 0. 6 t 250 600 3, 620 2. 9

l R.T. room temperature.

The tensile test results recorded in the foregoing table are each the average of the plurality of individual tests performed on identical test specimens cut from the same wire which had received the indicated treatment.

Upon comparison of these test results it will be noted that the hydrogen treatment of these lithium alloys invariably improved their yield strength and ultimate tensile strength at elevated temperature. For example, observe that the yield strength and the ultimate tensile strength at 250 C. of the 4% lithium alloy which had only been annealed at 325 C. for 2 hours in argon was improved by over 50% when this treatment was supplemented by an anneal at 560 C. for four hours in hydrogen, during which time substantially all the lithium was converted to lithium hydride, while the same material annealed at 560 C. in helium showed virtually no improvement in strength.

From these and other tests, we have found that for similar alloys containing less than about 0.5% lithium, no measurable improvement in mechanical properties is attained by the hydrogen annealing treatment. Also, we have found that when more than about 30% lithium is added, the subsequent hydriding treatment causes cracks to form in the alloy, presumably because of the dimensional changes which occur when the lithium hydride is formed.

From the foregoing it is apparent that alloys according to our invention, because of their ductility, may be readily fabricated in the final form and then hardened by a hydriding anneal. This may be accomplished by heating the formed article or stock material in a hydrogen atmosphere. Alternatively, the stock material, such as sheets and formed structural load bearing members, such as extruded beams, for example, may be assembled into finished form, perhaps with elements made from other structural alloys, and the whole assembly heat treated in a hydrogen atmosphere to harden the magnesium-lithium alloy components thereof. It is contemplated that complete structural assembly, such as an aircraft wing, for example, may be advantageously made in this manner. Furthermore, as previously pointed out, previously known magnesium-rich precipitation hardened alloys containing about or more magnesium may be improved by the addition of lithium according to our invention. First, the ductility of these alloys will be improved during the working stages of their fabrication and the subsequent precipitation of lithium hydride during the hydrogen anneal will act to increase the strengthening effect of the precipitates from the other alloying elements. Furthermore, since the lithium hydride precipitate is formed only by the reaction between the lithium which is completely in solution in the magnesium at the annealing temperature and the hydrogen which is diffused into the alloy, and it is contemplated that the anneal will be continued long enough so that substantially all the lithium is converted to hydride, excessive particle growth or overaging is inhibited during subsequent use at elevated temperatures in ordinary atmospheres since there is substantially no hydrogen available for reaction nor is there any unreacted lithium present even if sufiicient amounts of hydrogen are present. Since lithium hydride is a stable solid up to its melting point of about 680 C., the precipitated dispersion is eflFective as a hardener at elevated temperatures approaching the melting point of the magnesium alloy matrix.

Since the hydriding treatment depends upon the diffusion of hydrogen into the solid alloy, it will be apparent that the length of time required to completely transform the lithium to lithium hydride will depend upon the temperature, pressure or partial pressure of the hydrogen, the distance the hydrogen must diffuse into the metal body and the amount of lithium present to be reacted.

'It has been found that the annealing temperature may be advantageously maintained between about 350 C. to just below the melting point of the alloy; however temperatures below 350 C. may be employed, but the rate of hydrogen diffusion will be decreased. While in the preceding description reference has been made to a hydrogen atmosphere, it will be appreciated that the hydriding atmosphere may contain other gases in addition to hydrogen which do not deleteriously affect the reaction, such as, for example, helium, argon, nitrogen and others, however substantially pure hydrogen is preferred.

In addition, the alloys of our invention are useful as light-weight shielding materials to prevent the passage of nuclear radiation therethrough. As stated previously, the problem involves slowing down the high energy neutrons, absorption of thermal neutrons and degradation of secondary gamma rays produced by neutron interactions. It has been found that lithium hydride has considerable advantage in this regard in that it combines the high neutron slowing down power of hydrogen with the high neutron absorption of Li isotope which occurs as 7.5% of natural lithium. The absorption reaction is Li +n He +H therefore no secondary gamma rays are produced in the absorption of the neutrons. The cross-section for the reaction is 950 barns at thermal neutron energies. As will be appreciated by those skilled in the art, the Li content of natural lithium may be enriched if desired.

As previously pointed out, lithium hydride is a stable solid which melts at 680 C., but its mechanical properties are not high and it must be formed by powder techniques or cast directly to shape, and its thermal conductivity is lower than that of most metals.

According to our invention, articles composed of alloys of magnesium with up to 30% by weight lithium (about 60 atomic percent lithium) may be formed, either by casting directly to the desired shape or cast and subsequently worked to shape, and the article heat treated in hydrogen to convert substantially all the lithium to hydride. If the shielding element to be formed from this material is to be utilized as a load carrying member, it is preferred that the alloy contain less than 15% lithiurn. However, if it is to merely act as a shield, a higher lithium content may be advantageously employed.

This material has several advantages over pure lithium hydride. First, it is easier to fabricate. Second, load carrying members may be constructed therefrom which have considerable strength. Third, since the neutron absorption reaction set forth above generates substantial amounts of heat, the high thermal conductivity of the magnesium matrix permits extraction of the heat from the shield body and reduces the danger of exceeding the melting point of the lithium hydride at localized hot spots."

From the foregoing it will be seen that we have invented a new and useful structural and neutron shielding material and a method for making such a material, the scope of which is defined in the appended claims. While other modifications may readily occur to those skilled in the art, we do not intend our invention to be limited to the precise embodiment set forth in the preceding description, or in any other way except as set forth by the following claims.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. A method for improving the high temperature strength of an alloy consisting essentially of from about to about 99.5% by weight magnesium and from about 0.5% to about 30% by weight of lithium based on the magnesium content, comprising heat treating said lithium containing alloy in solid form in a hydriding atmosphere until substantially all the lithium is converted to lithium hydride.

2. A method for improving the high temperature strength of articles formed from an alloy consisting essentially of from about 60% to about 99.5% by weight magnesium and from about 0.5% to about 30% by weight of lithium based on the magnesium content, comprising melting such an alloy, forming a solid article from said alloy, and heat treating said article in a hydriding atmosphere until substantially all the lithium is converted to lithium hydride.

3. A method for improving the high temperature strength of articles formed from an alloy consisting essentially of up to 12% aluminum, up to 12% zinc, up to 5% tin, up to 5% silver, up to 20% cadmium, up to 1.5% nickel, up to 1.5% copper, up to 1.5% manganese, and the balance substantially all magnesium, comprising the steps of melting such an alloy, alloying from about 0.5 to 30% by weight lithium based on the magnesium content of said alloy with said molten alloy, forming a solid article from said alloy, and heat treating said article in a hydriding atmosphere until substantially all the lithium is converted to lithium hydride.

References Cited in the file of this patent Masing and Tammann: Ztsch. f. Anorg. Chem. (1910), 67, 183-199; article titled-The Behavior of Lithium Towards Sodium, Potassium, Tin, Cadmium, and Magnesium.

O. H. Henry and RV. Cordiano: The Lithium-Magnesium Equilibrium Diagram. Transactions of the A.I.M.E. (Institute of Metals Division), vol. 111, pages 319-31, 1934. 

1. A METHOD FOR IMPROVING THE HIGH TEMPERATURE STRENGTH OF AN ALLOY CONSISTING ESSENTIALLY OF FROM ABOUT 60% TO ABOUT 99.5% BY WEIGHT MAGNESIUM AND FROM ABOUT 0.5% TO ABOUT 30% BY WEIGHT OF LITHIUM BASED ON THE MAGNESIUM CONTENT, COMPRISING HEAT TREATING SAID LITHIUM CONTAINING ALLOY IN SOLID FORM IN A HYDRIDING ATMOSPHERE UNTIL SUBSTANTIALLY ALL THE LITHIUM IS CONVERTED TO LITHIUM HYDRIDE. 